It is well known to relay messages from one station to another through a relay station. Such arrangements are of particular importance when the range of, say, a mobile transmitting and/or receiving station is relatively small compared to that of the base station, or in areas where signal transmissions are obstructed or impaired. In such systems the general mode of operation is for the relay station simply to automatically repeat all messages.
In one such repeater arrangement wherein two frequencies are used, the base station transmits and receives on both frequencies. This system uses a relay station that receives on, say, the first frequency and automatically retransmits on the second frequency, whereas the mobile stations each receive on the second frequency and transmit on the first frequency. In operation, the base station transmits the same message on both frequencies, the first of which is received by the relay station and automatically retransmitted on the second frequency, while the mobile stations if within the range of the base station, receives the message transmitted on the second frequency from both the base station and the relay station. The relay will operate in the reverse in the event the mobile transmits a message on the second frequency. In this case, the relay will repeat that message and retransmit again on the second frequency, this time for the benefit of the base station. When the mobile is in range of the base the base will receive the message from both the mobile and the relay stations, but if out of range only the repeated message from the relay station will be received. Such a system is described in U.S. Pat. No. 2,642,524 issued June 16, 1953 to Bayliss.
Another known system is described in U.S. Pat. No. 3,745,462 issued July 10, 1973 to Trimble. This patent describes a mobile radio extension unit to permit the mobile radio to act as a relay station for say a hand held walkie-talkie or the like unit to permit the walkie talkie to communicate with a base station through the mobile transceiver. In this arrangement suitable controls are provided in the extension unit to interface the same with the mobile transceiver and the portable transceiver, such as a walkie-talkie. The equipment includes means to ensure that only one mobile transceiver receiving signals from the portable transceiver or from the base station retransmits the information to the receiving unit.
None of these prior art units provide a solution to the problem of data transmission between a base station and a mobile station and vice versa in areas where the airways are very crowded and very limited time and frequencies are available to carry the message. In such circumstances, which are now relatively common, it is undesirable to retransmit a message to the base station unless it is clear that the message has not been received, otherwise the airways or frequencies will be bombarded with extra, unnecessary messages and the time available, and thus the number of different messages that can be transmitted on a given frequency will be severely reduced.